It is desirable to detect a misfire condition (or a knock condition) during operation of an internal combustion engine. One approach taken in the art to detect misfire involves assessing the degree of engine speed (RPM) variation. Although such an approach does not require additional circuit components, it does require the availability of a microprocessor or the like and relatively extensive software to perform the evaluation, and, further, is incapable of producing accurate and reliable results over the entire engine speed and load range. In addition, one approach taken in the art for detecting knock involves the use of vibration knock sensors. Such an approach, however, involves, additional circuitry, including the sensors, which may be undesirable in certain circumstances.
In addition, so-called ion sense systems for detecting a combustion condition (e.g., misfire) are known. The combustion of an air/fuel mixture in an engine results in molecules in the cylinder being ionized. Applying a relatively high voltage across, for example, the electrodes of a spark plug just after the ignition operation is known to produce a current across the electrodes. Such current is known as ion current. The ion current that flows is proportional to the number of combustion ions present in the area of, for example, the spark plug gap referred to above, and is consequently indicative of the ionization throughout the entire cylinder as combustion occurs. The level or amount of ion current is indicative of a quality of the combustion event, or whether in fact combustion has occurred at all (e.g., a misfire condition). The level of an AC component of the ion current (e.g., at particular frequencies) may be used to determine knock.
Known ion current sensing systems generally include, in addition to an ignition coil, a capacitor or the like configured to store a voltage. The stored voltage is thereafter used as a "bias" voltage which is applied to the spark plug to generate the ion current. It is desirable to apply the bias voltage to the plug from the low voltage side of the secondary winding, rather than the high voltage side, to reduce the usage of high-voltage rated components. Accordingly, such biasing circuits are known that apply the biasing voltage to the low voltage side of a secondary winding of the ignition coil. However, known biasing circuits of this type incur substantial losses in spark energy. In particular, the bias voltage should be approximately 80-200 volts, and in one approach, the storage capacitor is disposed in parallel with a zener diode (having a reverse breakdown voltage in that range) between the low voltage end of the secondary and ground. Thus, during spark, the zener dissipates a relatively large amount of the spark energy (e.g., V.sub.ZENER *I.sub.SPARK) Compare this with the energy delivered by the plug (e.g., V.sub.SPARK *I.sub.SPARK where V.sub.SPARK may be as low as 200 volts), and it may be seen that as much as 1/3 to 1/2 of the spark energy is lost.
There is therefore a need to provide an apparatus for detecting a combustion condition such as a misfire condition or a knock condition, particularly a biasing circuit for use in an ion sense system, that minimizes or eliminates one or more of the shortcomings as set forth above.